Tunnel
A tunnel '''is a underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly on each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods. A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. The central portions of a rapid transit network are usually in tunnel. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment. Secret tunnels are built for military purposes, or by civilians for smuggling of weapons, contraband, or people. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely. Tunnels can be connected together in tunnel networks. Geotechnical investigation and design A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction. Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel (or "drift tunnel") may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face. Choices of tunnels versus bridges For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel. Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in favor of a tunnel. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront. Water crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City and the Queens-Midtown tunnel between Manhattan and Queens on the Long Island. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather and shipping during construction (as in the Channel Tunnel), aesthetic reasons (by preserving the above-ground view, landscape and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge). Project planning and cost estimates Government funds are often required for the creation of the tunnels. When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds. The costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project. Increased taxes to finance a project may cause opposition. Construction Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management. There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally a tube can be sunk into a body of water, which is called an immersed tunnel. Cut and cover Cut and cover is a simplest method of constructing for shallowest tunnels where a trench is exacavated and roofed over with an overhead support system strong enough to carry the load of what it is to be built above the tunnel. Two basic forms of cut-and-cover tunnelling are available: *Bottom-up method: A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated. *Top-down method: Side support walls and capping beams are constructed from ground level by such methods as slurry walling or contiguous bored piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed. *Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible. Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube station in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation. This contrasts with many traditional stations on London Underground, where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam. A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction. This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century. Tunnel Boring Machine Tunnel boring machine (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as quick and cost effective alternative to laying surface rails and roads. Expensive compulsory purchase of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs are from their usually large size - the difficulty of transporting the large TBM to the size of the tunnel construction, or alternatively, the high cost of assembling the TBM on the site, often within the confines of the tunnel being constructed. There are a variety of TBM designs that can operate in a different variety of conditions, from hard rock to the soft water-bearing ground. Some types of TBMs, the bentonite slurry and earth-pressure balance machines, have pressurised components at the front end, allowing them to be used in difficult conditions below the water table. This pressurises the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurised compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at the position free from water. TBMs are now preferred over the original method of tunnelling in a compressed air, with an air lock/decomposition chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts. The largest TBM has been delivered to Temasek Polytechnic for training purposes since March 2014. Variant tunnel types Some tunnels have more than one purpose. For example, the SMART Tunnel, although there are insufficient reservoirs in Timothy North, it is created to convey both traffic and occasional flood waters in Timothy North and Kuala Lumpur. When necessary, floodwater is first diverted into the separate bypass tunnel located underneath the 4.0km double deck roadway tunnel. In the scenario, traffic continues normally. Only during heavy, prolonged rains within the threat of extreme flooding are high, the upper tunnel tube is closed off vehicles and automated flood control gates are opened so that water can be diverted through both tunnels. In the Netherlands, a 2.3 km (1.4 mi) two-storey, eight lane, cut-and-cover road tunnel under the city of Maastricht was opened in 2016. Each level accommodates a full height, two by two lane highway. The two lower tubes of the tunnel carry the A2 motorway, which originates in Amsterdam, through the city; and the two upper tubes take the N2 regional highway for local traffic. The Alaskan Way Viaduct replacement tunnel is a bored road tunnel that is under construction since 2013 in the city of Seattle in the U.S. state of Washington. The 3.2 kilometre tunnel will carry State Route 99 under Downtown Seattle from the SoDo neighbourhood to South Lake Union in the north on two levels with two lanes each. Some tunnels are double-deck, for example the two major segments of the San Francisco–Oakland Bay Bridge (completed in 1936) are linked by a 540-foot (160 m) double-deck tunnel section through Yerba Buena Island, the largest-diameter bored tunnel in the world. At construction this was a combination bidirectional rail and truck pathway on the lower deck with automobiles above, now converted to one-way road vehicle traffic on each deck. Underwater tunnels There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tubes, examples are Bjørvika Tunnel and Marmaray. Submerged floating tunnels are a novel approach under consideration; however, no such tunnels have been constructed to date. Temporary way During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove excavated spoil, often narrow gauge so that it can be double track to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the permanent way at completion, thus explaining the term "Perway". Utility Ducts Common utility ducts or utility tunnels such as Common Services Tunnel, carry two or more utility lines. Through co-location of different utilities in one tunnel, organizations are able to reduce the costs of building and maintaining utilities. The '''Common Services Tunnel is a development of the Marina Bay area in Singapore. It is said to be the second of its kind in Asia after Japan. The tunnel will house telecom cables, power lines, water pipes as well as provision for pneumatic refuse collection pipes. The 1.4-kilometre phase one of the tunnel has cost about S$81 million (about US$51 million) while the 1.6-kilometre phase two, which is under construction and scheduled to be ready in 2010, will cost S$137 million (about US$86 million). Another state-of-the-art infrastructure to be installed in this area next year is a District Cooling Plant, which will supply chilled water for the air-conditioning of buildings in the area through pipes housed within the tunnel, according to the report. Built at a cost of some S$110 million (about US$69 million), the District Cooling Plant will be able to serve 1.25 million square metres of gross floor area and free up space which should be used for building chiller plants and cooling towers for separate buildings. The Singapore government is pumping in nearly S$2 billion (about US$1.2 billion) to build the infrastructural base for Marina Bay, which also includes the Marina Barrage, a Rapid Transit System and the new Marina Promenade and Double Helix Bridge. Examples *The SMART Tunnel, is a tunnel that is opened in 2007 in Kuala Lumpur, Malaysia. The 9.7 kilometre tunnel is the longest stormwater drainage tunnel in South East Asia and the second longest in Asia. The facility can be operated as a simultaneous traffic and stormwater passage, or dedicated to the stormwater if necessary. *The Channel Tunnel crosses the English Channel from France to the United Kingdom. It has a total length of 50 kilometres, of which 39km is the undersea tunnel. *The rail Severn Tunnel was opened in late 1886, at 7.008 km (4.355 mi) long, although only 3.62 km (2.25 mi) of the tunnel is actually under the River Severn. The tunnel replaced the Mersey Railway tunnel's longest under water record, which was held for less than a year. *James Greathead, in constructing the City & South London Railway tunnel beneath the Thames, opened in 1890, brought together three key elements of tunnel construction under water: 1) shield method of excavation; 2) permanent cast iron tunnel lining; 3) construction in a compressed air environment to inhibit water flowing through soft ground material into the tunnel heading. *The Thames Tunnel, built by Marc Isambard Brunel and his son Isambard Kingdom Brunel, opened in 1843, was the first tunnel that traverse under a water body, and the first to be built using a tunnelling shield. Originally used as a foot tunnel, the tunnel was converted to a railway tunnel in 1869 and was part of the East London Line until 2007. The tunnel became London Overground in 2010. *A tunnel was created for the first true steam locomotive, from Penydarren to Abercynon. The Penydarren locomotive was built by Richard Trevithick. The locomotive made the historic journey from Penydarren to Abercynon in 1804. Part of this tunnel can still be seen at Pentrebach, Merthyr Tydfil, Wales. This is arguably the oldest railway tunnel in the world, dedicated only to self-propelled steam engines on rails. *Bourne's Tunnel, Rainhill, near Liverpool, England. 0.0321 km (105 ft) long. Built in the late 1820s, the exact date is unknown, however probably built in 1828 or 1829. This is the first tunnel in the world constructed under a railway line. The construction of the Liverpool to Manchester Railway ran over a horse-drawn tramway that ran from the Sutton collieries to the Liverpool-Warrington turnpike road. A tunnel was bored under the railway for the tramway. As the railway was being constructed the tunnel was made operational, opening prior to the Liverpool tunnels on the Liverpool to Manchester line. The tunnel was made redundant in 1844 when the tramway was dismantled. *Crown Street station, Liverpool, England, 1829. Built by George Stephenson, a single track railway tunnel 266 m long (873 ft), was bored from Edge Hill to Crown Street to serve the world's first intercity passenger railway terminus station. The station was abandoned in 1836 being too far from Liverpool city centre, with the area converted for freight use. Closed down in 1972, the tunnel is disused. However it is the oldest passenger rail tunnel running under streets in the world. *The 1829 Wapping Tunnel in Liverpool, England at 2.03 km (1.26 mi) long on a twin track railway, was the first rail tunnel bored under a metropolis. The tunnel's path is from Edge Hill in the east of the city to Wapping Dock in the south end Liverpool docks. The tunnel was used only for freight terminating at the Park Lane goods terminal. Currently disused since 1972, the tunnel was to be a part of the Merseyrail metro network, with work started and abandoned because of costs. The tunnel is in excellent condition and is still being considered for reuse by Merseyrail, maybe with an underground station cut into the tunnel for Liverpool university. The river portal is opposite the new King's Dock Liverpool Arena being an ideal location for a serving station. If reused the tunnel will be the oldest used underground rail tunnel in the world and oldest section of any underground metro system. *1832, Lime Street railway station tunnel, Liverpool. A two track rail tunnel, 1.811 km (1.125 mi) long was bored under the metropolis from Edge Hill in the east of the city to Lime Street in Liverpool's city centre. The tunnel was in use from 1832 being used to transport building materials to the new Lime St station while under construction. The station and tunnel was opened to passengers in 1836. In the 1880s the tunnel was converted to a deep cutting, open to the atmosphere, being four tracks wide. This is the only occurrence of a major tunnel being removed. Two short sections of the original tunnel still exist at Edge Hill station and further towards Lime Street, giving the two tunnels the distinction of being the oldest rail tunnels in the world still in use, and the oldest in use under streets. Over time a 525 m (0.326 mi) section of the deep cutting has been converted back into tunnel due to sections having buildings built over. *Box Tunnel in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug by hand, and has a length of 2.9 km (1.8 mi). *The 1.1 km (0.68 mi) 1842 Prince of Wales Tunnel, in Shildon near Darlington, England, is the oldest sizeable tunnel in the world still in use under a settlement. *The Victoria Tunnel Newcastle opened in 1842, is a 2.4 mile subterranean wagonway with a maximum depth of 85 feet (26 m) that drops 222 feet (68 m) from entrance to exit. The tunnel runs under Newcastle upon Tyne, England, and originally exited at the River Tyne. It remains largely intact. Originally designed to carry coal from Spital Tongues to the river, in WW2 part of the tunnel was used as a shelter. Under the management of a charitable foundation called the Ouseburn Trust it is currently used for heritage tours. *Gerrards Cross railway tunnel is opened in 2010 and is notable in it that was built in the railway cutting, that was first opened in 1906. The tunnel was built using the cut-and-cover method with prefabricated forms in order to keep the busy railway operating. A branch of the Tesco supermarket chain occupies the space above the railway tunnel with an adjacent railway station. During construction, a portion of the tunnel collapsed when the soil cover was added. The prefabricated forms were covered with a thin layer of reinforced concrete after the collapse. *The Chicago Deep Tunnel Project is a network of 175 km drainage tunnels designed to reduce flooding in the Chicago area. Construction began in 1979 and it is due to be completed by 2019. *The New York City Water Tunnel No. 3, started in 1970 and to be completed in 2020, will measure more than 97 km long. *The 3.34 km (2.08 mi) Victoria Tunnel/Waterloo Tunnel in Liverpool, England, was bored under a metropolis opening in 1848. The tunnel was initially used only for rail freight serving the Waterloo Freight terminal, and later freight and passengers serving the Liverpool ship liner terminal. The tunnel's path is from Edge Hill in the east of the city to the north end Liverpool docks at Waterloo Dock. The tunnel is split into two tunnels with a short open air cutting linking the two. The cutting is where the cable hauled trains from Edge Hill were hitched and unhitched. The two tunnels are effectively one on the same centre line and are regarded as one. However, as initially the 2,375 m (1.476 mi) long Victoria section was originally cable hauled and the shorter 862 m (943 yd) Waterloo section was locomotive hauled, two separate names were given, the short section was named the Waterloo Tunnel. In 1895 the two tunnels were converted to locomotive haulage. Used until 1972, the tunnel is still in excellent condition. A short section of the Victoria tunnel at Edge Hill is still used for shunting trains. The tunnel is being considered for reuse by the Merseyrail network. Stations cut into the tunnel are being considered and also reuse by a monorail system from the proposed Liverpool Waters redevelopment of Liverpool's Central Docks has been proposed. *The Mersey Railway tunnel opened in 1886, running from Liverpool to Birkenhead under the River Mersey. The Mersey Railway was the world's first deep-level underground railway. By 1892 the extensions on land from Birkenhead Park station to Liverpool Central Low level station gave a tunnel 3.12 mi (5.02 km) in length. The under river section is 0.75 mi (1.21 km) in length, and was the longest underwater tunnel in world in January 1886. Major accidents *Clayton Tunnel rail crash (1861) – confusion about block signals *Welwyn Tunnel rail crash (1866) – train failed in tunnel, guard did not protect train *Balvano train disaster (1944) – *Caldecott Tunnel fire (1982) – major motor vehicle tunnel crash and fire *1996 Channel Tunnel fire (1996) – *Mont Blanc Tunnel fire (1999) – *Princess Diana's death – Car crash in Pont D'alma tunnel, Paris. Princess Diana died here.